Bactericidal silicon dioxide doped with silver

ABSTRACT

Pyrogenically prepared silica doped with silver or silver oxide is prepared by feeding an aerosol into a flame such as is used for the preparation of pyrogenic silica, mixing the aerosol homogeneously with gas mixture before the reaction, then allowing the aerosol/gas mixture to react in a flame. The resulting pyrogenic silicas doped with silver or silver oxide are separated from the gas stream. The pyrogenic silica doped with silver or silver oxide by means of an aerosol can be used as a bactericidal filler.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based on German Application DE 199 43 057.8,filed Sep. 9, 1999, which disclosure is incorporated herein byreference.

FIELD OF THE INVENTION

[0002] The invention relates to a pyrogenically prepared silicon dioxidedoped with silver or silver oxide by means of an aerosol and havingbactericidal properties, and to a process for its preparation and itsuse. The invention relates also to use of this doped silicon dioxide indispersions or as a filler in rubber and silicone rubber.

BACKGROUND OF THE INVENTION

[0003] It is known to dope pyrogenically prepared silica in one step ina flame using a special process. (DE 96 50 500 A1). That process is acombination of high-temperature flame hydrolysis with pyrolysis. Thatdoping process is to be distinguished from the older, so-called“co-fumed process”, in which the gaseous starting materials (for exampleSiCl₄ gas and AlCl₃ gas) are pre-mixed and burnt together in a flamereactor.

SUMMARY OF THE INVENTION

[0004] In the doping process, an aerosol is fed into a flame in which apyrogenic oxide is produced by flame hydrolysis, the aerosol containinga salt of the compound to be doped.

[0005] It has now been found that if silver salts dissolved in water areused as the starting material for the aerosol, a doped pyrogenic silicahaving bactericidal properties is obtained as the product.

[0006] The invention provides a pyrogenically prepared silica doped withsilver or silver oxide by means of an aerosol. The silica ischaracterized in that the base component is a silica preparedpyrogenically by flame oxidation or, preferably, by flame hydrolysis.The silica is doped with a doping component of from 0.0001 wt. % up to20 wt. %, the doping amount preferably being in the range from 1 to10,000 ppm, and the doping component is a salt or a salt mixture of asilver compound or metallic silver or mixtures thereof. The BET surfacearea of the doped oxide is from 1 to 600 m²/g, preferably in the rangefrom 40 to 400 m²/g.

[0007] The invention further provides a process for the preparation ofthe pyrogenically prepared silica doped with silver or silver oxide bymeans of an aerosol, which process is characterized in that an aerosolis fed into a flame such as is used for the pyrogenic preparation ofsilica by flame oxidation or, preferably, by flame hydrolysis, theaerosol is mixed homogeneously with the gas mixture used for the flameoxidation or the flame hydrolysis before the reaction, then theaerosol/gas mixture is allowed to react in the flame and the resultingpyrogenically prepared silicas doped with silver or silver oxide areseparated from the gas stream in a known manner. An aqueous solutioncontaining salts or salt mixtures of silver or the metal itself indissolved or suspended form or mixtures thereof is used for thepreparation of the aerosol. The aerosol is produced by atomization bymeans of a two-component nozzle or by a different method of aerosolproduction.

[0008] The following may be used as salts: Ag(NO₃), Ag₂(SO₄), Ag₂O, orthe salts may be complexed with complexing agents or ammonia.

[0009] The process of flame hydrolysis for the preparation of pyrogenicoxides is known from Ullmanns Enzyklopädie der techn. Chemie 4thedition, Vol. 21, p. 464.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic representation of the doping apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The doped silicon dioxide according to the invention and theprocess for its preparation and use are described below with referenceto FIG. 1 and the following Example.

[0012] The core element of the apparatus is a burner of known type forthe preparation of pyrogenic oxides.

[0013] The burner 1, shown in FIG. 1 includes a central pipe 2 whichopens into a nozzle 3 from which the main gas stream flows into theburner chamber 10,in which chamber the gas burns. This inner nozzle 3 issurrounded by a ring nozzle or jacket 4, from which secondary hydrogenflows.

[0014] In the central pipe 2 there is a further axial pipe 5, which endsseveral centimeters before the outlet of the nozzle 3 of the centralpipe 2. The aerosol is fed into the axial pipe 5.

[0015] The aerosol, which is an aqueous silver salt solution, isproduced in an aerosol generator 6, which may be an ultrasonic atomizer.

[0016] The silver salt/water aerosol produced in the aerosol generatoris passed by means of a light carrier gas stream through a heating zone7, in which the entrained water vaporizes, leaving small salt crystalsin finely divided form in the gas phase.

[0017] Secondary air is fed into the burner chamber through inlet 8.

EXAMPLE 1 Preparation of a Pyrogenic Silica Doped with Silver or SilverOxide

[0018] 4.44 kg/h of SiCl₄ are vaporized at about 130° C. and transferredto the central pipe of a burner of known type. 3 Nm³/h of primaryhydrogen and 7.5 Nm³/h of air are additionally fed into the centralpipe.

[0019] The gas mixture flows out of the inner nozzle of the burner andburns in the burner chamber and the water-cooled flame tube adjacentthereto.

[0020] 0.5 Nm³/h of secondary hydrogen are fed into the ring nozzle.

[0021] 12 Nm³/h of secondary air are additionally fed into the burnerchamber.

[0022] A second gas stream flows out of the axial pipe 5 into thecentral pipe 2.

[0023] The second gas stream consists of an aerosol which is produced ina separate atomizing unit by ultrasonic atomization. The aerosolgenerator atomizes 725 g/h of 5% aqueous silver sulfate solution. Thesilver sulfate aerosol is guided through a heated line with the aid ofthe carrier gas of 0.5 Nm³/h of air, the aqueous aerosol being convertedat temperatures of about 180° C. into a gas and a salt crystal aerosol.

[0024] The reaction gases and the resulting pyrogenic silica doped withsilver or silver oxide are drawn through a cooling system by theapplication of reduced pressure, and the particle gas stream therebycools to a range of about 100° to 160° C. The solid is separated fromthe waste gas stream in a filter or cyclone (not shown).

[0025] The pyrogenically prepared silica doped with silver or silveroxide is obtained in the form of a finely divided white powder. In afurther step, any hydrochloric acid residue still adhering to the silicais removed therefrom at elevated temperature by treatment with aircontaining water vapor.

[0026] The BET surface area of the pyrogenic silica doped with silver is206 m²/g.

[0027] The preparation conditions are summarized in Table 1, furtheranalytical data of the silica so obtained are given in Table 2. TABLE 1Experimental conditions for the preparation of pyrogenic silica dopedwith silver or silver oxide Pri- mary Sec. H₂ Aero- air air core H₂ solAir SiCl₄ Nm³/ Nm³/ Nm³/ jacket Salt amount aerosol BET kg/h h h h Nm³/hsolution kg/h Nm³/h m²g 4.44 7.5 12 3 0.5 5% 0.725 0.5 206 aqueousAg₂SO4

[0028] TABLE 2 Analytical data Tamped Ag₂O BET pH value density contentm²/g 4% sus. g/l wt. % Example 206 4.13 24 1.72

[0029] Advantages of the silica according to the invention:

[0030] The silica according to the invention doped with silver or silveroxide has bactericidal properties.

[0031] The pyrogenic silica doped with silver or silver oxide by meansof an aerosol can be used as a bactericidal filler especially in theproduction of paints or coatings, in the paper industry, as acatalytically active substance, as a starting material for thepreparation of dispersions, as a polishing material (CMP applications),as a ceramics base material, in the electronics industry, as a fillerfor polymers, as a starting material for the production of glass orglass coatings or glass fibers, as a release auxiliary even at hightemperatures, in the cosmetics industry, as an absorber material, as anadditive in the silicone and rubber industry, for adjusting the rheologyof liquid systems, for heat protection stabilization, as a heatinsulating material, as a flow auxiliary, as a filler in the dentalindustry, as an adjuvant in the pharmaceuticals industry or in the paintindustry.

What is claimed is:
 1. Pyrogenically prepared silica doped with silveror silver oxide, comprising silica prepared pyrogenically by flameoxidation or flame hydrolysis, wherein said silica is doped with adoping component in an amount from 0.0001 wt. % to 20 wt. %, the dopingcomponent being at least one member selected from the group consistingof silver salts, silver salt mixtures, suspension of a silver compound,suspension of metallic silver and mixtures thereof, wherein the BETsurface area of the doped oxide is from 1 to 600 m²/g.
 2. Pyrogenicallyprepared silica according to claim 1, wherein the doping amount is from1 to 10,000 ppm.
 3. A pyrogenically prepared silica according to claim1, wherein the BET surface area is from 40 to 400 m²/g.
 4. A process forthe preparation of pyrogenic oxides doped with silver or silver oxide,comprising: preparing an aerosol using as starting material a saltsolution containing salts or salt mixtures of silver or silver metal indissolved or suspended form or mixtures thereof, the aerosol beingproduced by atomization by means of a two-component nozzle; mixing theaerosol homogeneously with a gas mixture used for flame oxidation orflame hydrolysis; feeding the mixture of the aerosol and gas into aflame used for preparation of pyrogenic oxides by flame oxidation orflame hydrolysis, reacting the mixture of aerosol and gas in the flame;and separating pyrogenic oxides doped with silver or silver oxide from aresulting gas stream.
 5. A process according to claim 4, wherein thepyrogenic oxide comprise pyrogenic silica.
 6. A process according toclaim 4, wherein the aerosol is produced by an aerosol generator.
 7. Aprocess according to claim 6, wherein the aerosol generator comprises anultrasonic atomizer.